1. Field of the Invention
The present invention relates to novel chiral copolymers with oligosiloxane spacers and the use of such polymers as chiral stationary phases in analytical and preparative gas, supercritical fluid and liquid chromatographic separations. More particularly, this invention relates to novel chiral copolymers which contains chiral molecular cavities or chiral divalent linear molecules which could have a secondary structure possessing molecular grooves for use in chiral stationary phase separations. This invention also relates to the use of these copolymers for analysis of enantiomeric and other stereoisomeric mixtures of various substances. This invention relates as well to the use of either open tubular column surface-bonded or packed column particle-bound chiral copolymeric siloxane materials, prepared by coating and immobilizing by chemical reaction or heating the chiral copolymeric siloxanes on the desired column or particle surface for gas, supercritical fluid and liquid chromatography separations, and the separation of enantiomeric and other stereoisomeric mixtures of various structures using these materials.
2. Prior Art
Chiral chromatographic methods have become important as methods for determining enantiomeric excess and for preparatively resolving enantiomers. General references which discuss chiral phases and their uses are as follows: (1) W. A. Konig, "Separation of Enantiomers by Capillary Gas Chromatography with Chiral Stationary Phases," J. Resolut. Chromatogr./Chromatogr. Commun. (1982), v. 5, pp. 588-595; (2) R. H. Liu et al., "Chiral Stationary Phases for the Gas Liquid Chromatographic Separation of Enantiomers," J. Chromatogr. (1983), v. 271, pp. 309-323; (3) B. Koppenhoefer et al., "Chiral Recognition in Gas Chromatographic Resolution of Enantiomers on Chiral Polysiloxanes," J. Chromatogr. Library (1985), V. 32, pp. 1-42; (4) D. W. Armstrong et al., "Polar-Liquid, Derivatized Cyclodextrin Stationary Phases for the Capillary Gas Chromatography Separation of Enantiomers," Anal. Chem. (1990), v. 62, pp. 914-923; (5) K. Nakamura et al., "Chiral Polysiloxanes Derived from (R,R)-Tartaramide for the Gas Chromatographic Separation of Enantiomers," Anal. Chem. (1989), v. 61, pp. 2121-2124; (6) K. Nakamura et al., "Direct Resolution of Enantiomeric Diols by Capillary Gas Chromatography on a Chiral Polysiloxane Derived from (R,R)-Tartaramide," Anal. Chem. (1990), v. 62, pp. 539-541; and (7) V. Schurig and H.-P. Nowotny, "Gas Chromatographic Separation of Enantiomers on Cyclodextrin Derivatives," Angewandte Chemie International Edition in English (1990), V. 29, pp. 939-1076.
Three recent papers which describe the use of a chiral stationary phase in supercritical fluid chromatographic separations are (1) W. Roder et al., "Chiral SFC-Separations Using Polymer-Coated Open Tubular Fused Silica Columns. Comparison of Enantiomeric Selectivity in SFC and LC Using the Same Stationary Phase of the Pirkle Type," J. High Resolut. Chromatogr./Chromatogr. Commun. (1987), v. 10, pp. 665-667; (2) p. Macaudiere et al., "CO.sub.2 Supercritical Fluid Chromatography with Chiral Phases: A Promising Coupling for the Resolution of Various Racemates," J. Chromatogr. Sci. (1989), V. 27, pp. 383-394; and (3) B.E. Rossiter et al., "The Rational Design of Chiral Stationary Phases for Capillary Supercritical Fluid Chromatography," Tetrahedron Letters (1991), v. 32, pp. 3609-3612. This latter paper describes chiral copolymers composed of a chiral hydrocarbon containing C.sub.2 symmetry and an oligosiloxane spacer. These chiral copolymers are Similar to those described in this patent application except in the present case, the chiral portion of the copolymer is a chiral molecular groove or cavity.
Such methods rely on the presence of a chiral stationary phase (hereafter referred to as CSP) in the chromatographic system as a means of interacting with solutes such that one enantiomer of the solute emerges from the chromatographic system before the other, thus, separating the two enantiomers from each other. The first CSPs were developed for gas chromatography. Such CSPs were capable of resolving certain classes of racemates. They were limited in their utility in that the number of types of racemates they resolved was small and the CSPs, being low molecular weight organic compounds, had a pronounced tendency to bleed from the column even at relatively low temperatures (e.g., &lt;200.degree. C.). Thus they could resolve only a small number of volatile compounds. Since that time, effort directed towards the development of CSPs has largely had two objectives, i.e., to increase the selectivity of these materials so as to be able to separate the enantiomers of other types of racemates and to improve general chromatographic properties among which are thermal and chemical resiliency, low melting and high boiling points and high diffusion coefficients.
Because the mechanism for enantioselection Was and to a large degree still is incompletely understood, much of the effort directed towards CSP development centered more on an empirical effort to discover new types of functional group combinations capable of resolving enantiomers more than on incorporating desirable chromatographic properties. Even today, many types of CSPs for GC still have relatively poor chromatographic properties. Parallel development of achiral stationary phases for GC however demonstrated that the polysiloxanes have highly desirable characteristics for chromatographic separations. These qualities include low melting and high boiling points, high solute diffusion coefficients, and ease of properly distributing and immobilizing on chromatographic support materials. They can be anchored to fused silica surfaces in chromatographic columns through cross-linking processes, and can be deactivated, so as to promote extremely low volatility and high chemical and thermal stability. An important breakthrough came with the development of the CSP now known to as Chirasil-Val.TM.. This was first reported by H. Frank et al., "Rapid Gas-Chromatographic Separation of Amino Acid Enantiomers with a Novel Chiral Stationary Phase," J. Chromatogr. Sci. (1977), v. 15, pp. 174-179. This polymer incorporated features from both areas of development, i.e., desirable chromatographic properties and a pronounced ability to differentiate chromatographically different types of racemates. This material is characterized as being a polysiloxane possessing a certain percentage of pendant groups as chiral substituents whose purpose is to interact differentially with solutes. This CSP has proven to be very popular for the separation of enantiomers of certain types of compounds. Similar CSPs have since been developed.
In spite of the great benefits of Chirasil-Val.TM. and its sister CSPs, they have a major limitation. They are very specific with respect to the types of racemates they will resolve. The specificity of the CSP originates with the particular cluster of functional groups and the molecular structure of the chiral pendant arm. This limits their usefulness to the separation of only a limited number of enantiomers.
Certain alkylated cyclodextrins have been used for the separation of enantiomers in GC. These alkylated cyclodextrins have been diluted with polysiloxanes and coated on glass or fused silica columns as reported by H.P. Nowotny et al., "Extending the Scope of Enantiomer Separation in Diluted Methylated .beta.-Cyclodextrin Derivatives by High Resolution Gas Chromatography," J. High Resolut. Chromatogr. (1989), v. 12, pp. 383-393; or lipophilic pern-pentylcyclodextrins and other similar peralkyl substituted cyclodextrins have been coated directly onto the column surfaces as discussed extensively in the aforementioned review by Schurig and Nowotny. These techniques have allowed enantiomeric separations of a variety of organic materials in GC at temperatures of 25 to 250.degree. C. These materials have not been crosslinked or bonded to the glass or fused silica surfaces and so they cannot be used in GC at high temperatures (&gt;250.degree. C.) or in SFC or LC. In high temperature GC, these materials will bleed from the column surface and in SFC or LC, they will be washed from the column by the supercritical fluid or liquid.
Thus it will be recognized that what is needed in the art is an approach to CSP design that will produce CSP's with chiral resolving abilities, i.e. the ability to separate enantiomers and other stereoisomers, as well as excellent chromatographic properties, i.e. low and high operating temperatures, high solute diffusion with excellent separation properties at low temperatures, and that can be immobilized on a chromatographic column so that liquids or supercritical fluids will not strip or wash the phase off the columns. Similar phases which are stable to a wide variety of solvents and can be bound onto silica particles for liquid chromatography (LC) columns would also be an important addition to LC technology. Because it will be difficult or impossible to develop a single CSP which will resolve all classes of racemates, it is further desirable to be able to easily modify the CSP so as to accommodate different types of racemates while retaining desirable chromatographic properties. It is further desirable that the overall CSP design be such as to maximize the enantiomer resolving properties of the CSP. In this regard, it would be desirable to have these new CSPs contain chiral molecular grooves or molecular cavities, such as a cyclodextrin, which has a proven ability to separate all types of enantiomers, or chiral crown ethers or cyclams. Chromatographic phases having these novel properties for separating enantiomeric substances are disclosed and claimed in the present invention.